helper: New experimental CalculateAllPossibleRoutes method in ndn::GlobalRoutingHelper

Implementation is not time-efficient and it should not be used for very
large topologies.
diff --git a/helper/ndn-global-routing-helper.h b/helper/ndn-global-routing-helper.h
index 983c7b0..f233e89 100644
--- a/helper/ndn-global-routing-helper.h
+++ b/helper/ndn-global-routing-helper.h
@@ -98,9 +98,19 @@
   /**
    * @brief Calculate for every node shortest path trees and install routes to all prefix origins
    */
-  void
+  static void
   CalculateRoutes ();
 
+  /**
+   * @brief Calculate all possible next-hop independent alternative routes
+   *
+   * Refer to the implementation for more details.
+   *
+   * Note that this method is highly experimental and should be used with caution (very time consuming).
+   */
+  static void
+  CalculateAllPossibleRoutes ();
+
 private:
   void
   Install (Ptr<Channel> channel);